The present invention relates to a methods and apparatus for receiving radio frequency signals. More particularly, it relates to a method and apparatus for converting a radio frequency signal to a signal with a predetermined frequency without affecting its phase or amplitude measurement.
Transmission of a radio frequency signal usually requires multiple frequency conversion steps. For example, a low frequency signal is generally converted to a high frequency signal before it is transmitted by various means. After it is received, this converted high frequency signal is then down-converted to a signal of a same or similar low frequency for further processing. This approach is widely practiced in many areas, particularly in the area of telecommunication.
A radio frequency signal is a complex signal composed of several important parameters, including phase, amplitude, and frequency. One key aspect of the signal transmission process is how to preserve information concerning a signal's phase and amplitude. This is especially important when such a signal is being converted to another frequency. Over the past years, extensive research efforts have been made to minimize impact on a radio frequency signal's phase and amplitude information during the frequency conversion process.
There are a number of prior art approaches on how to preserve a radio frequency signal's phase and amplitude information while converting its frequency. Most prior art approaches employ one or multiple offset signals separately generated. These offset signals are then combined with test signals to provide signals at another frequency.
One example of these prior art approaches is a microwave receiver, herein briefly described in FIG. 1. It is generally used for the measurement of radar cross section. This microwave receiver employs a three-level frequency conversion process. It first comprises a local oscillator or similar devices which generate an offset signal. The combination of this offset signal with either a test signal or a reference signal gives rise to signals at a frequency of 2 GHz. These 2 GHz signals are then similarly converted to intermediate frequency signals at 45 MHz, which are further downconverted to a low frequency at 5.02 KHz for processing.
Another example is a network analyzer, herein briefly described in FIG. 2. It also uses local oscillators to generate offset signals. These offset signals are first combined with either a test signal or its reference signal to provide signals of an intermediate frequency at 20–27.8 MHz. These intermediate frequency signals are then converted to signals at a frequency of 278 KHz before being further processed.
In order to preserve information regarding a test signal's phase and amplitude, most prior art approaches attempt to convert a test signal, as well as its reference signal, to a precise frequency or a very narrow frequency range. But one problem is that most test signals, as well as their reference signals, are relatively unstable. In other words, their frequencies may be constantly fluctuating. This may be due to various reasons, such as limitation of signal sources or instability of power voltage.
To solve this problem, many prior art approaches employ a device called phase-locked loop (PLL), as shown in FIG. 1. One primary function of a PLL, as well as other similar devices, such as digital frequency synthesizers, is to provide a converted signal that maintains a constant phase angle relative to a reference signal just like the original, unconverted signal. The underlying mechanism is to control the offset signal's frequency so that it is constantly adjusted according to variations of a test signal's frequency. In this way, a test signal, as well as its reference signal, may be converted to a particular frequency or a narrow frequency range without affecting measurements of its phase and amplitude. Other approaches, as shown in FIG. 2, also employ devices, such as pass band filter, to facilitate this process so that only signals within a narrow frequency range may pass through.
The use of these devices, however, gives rise to several problems. First of all, because of their reliance on these devices, most prior art approaches either are incapable of handling signals with a wide frequency range, or they lack sensitivity, dynamic range or frequency agility generally required for measuring a radio frequency signal with precision. Meanwhile, the use of these devices inevitably increases systematic errors which may eventually affect the accuracy of measurements. Finally, most of these devices are complicated and expensive.